1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly relates to a semiconductor memory device having a function of access control on a relieved address.
2. Description of Related Art
With an increase in a memory capacity of semiconductor memory devices such as DRAM (Dynamic Random Access Memory), there is also an accompanying increase in the number of memory cells that do not function properly (hereinafter, “defective cells”). Therefore, in anticipation of generation of defective cells, auxiliary memory cells known as redundant cells are provided in semiconductor memory devices. Memory addresses of defective cells are relieved by switching memory addresses of the defective cells to memory addresses of the redundant cells. The address of the defective cell that should to be relieved is called “relieved address” in the following explanations. Detection of the defective cells and switching of memory addresses of the defective cells to the memory addresses of the redundant cells is performed in a wafer state during manufacturing of semiconductor memory devices. Specifically, defective cells are detected in an operation test performed in the wafer state, and if defective cells are detected, then their relieved addresses are programmed in a relieved address memory area provided in the semiconductor memory device. When the relieved address is a target of access, the target is changed to the redundant cell associated with the relieved address.
Nonvolatile memory elements in the form of fuse elements are often used for holding relieved addresses. The fuse elements are electrically conductive in their initial state, but are changed to a nonconductive state (an insulated state) due to path blockage caused by irradiation with a laser beam. Information equivalent to 1 bit is expressed according to the conductive/nonconductive state of the fuse element. Therefore, by selectively irradiating a plurality of the fuse elements with the laser beam, desired addresses can be recorded in a nonvolatile manner. The process of recording these relieved addresses is normally called “trimming” or “programming”. Semiconductor memory device having nonvolatile memory elements to store relieved addresses are described in Japanese Patent Application Laid-Open No. 2008-071407 and No. 2008-186847.
After a wiring process, a principal surface of a semiconductor chip is covered with a protective film called a passivation film. After covering, an operation test is performed to detect the defective cells and then the trimming is performed. To facilitate the trimming, openings are provided in advance on the passivation film positioned over fuse elements. The laser beam is irradiated on the fuse elements through these openings. A relief circuit arranged adjacent to a fuse element group reads the state (bit) of the fuse elements to specify the relieved address.
When providing a plural of openings, a certain margin must be provided between the openings. If this margin area is large, the number of the fuse elements that can be packed in a unit surface area (hereinafter, “integration density”) drops. Larger but fewer openings are preferable to smaller and more openings, because this configuration leads to an increased integration efficiency.
Furthermore, the relieved address is read out from the fuse element by the relief circuit, and supplied to a memory bank as a relief address signal. It is important to keep a signal path of the relief address signal simple to increase a transmission speed of the relief address signal.